This invention relates generally to electrophotographic imaging members and, more specifically, to layered photoreceptor structures with improved overcoat layers and processes for making the imaging members.
Electrophotographic imaging members, i.e. photoreceptors, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the dark so that electric charges can be retained on its surface. Upon exposure to light, the charge is dissipated.
An electrostatic latent image is formed on the photoreceptor by first uniformly depositing an electric charge over the surface of the photoconductive layer by one of the many known means in the art. The photoconductive layer functions as a charge storage capacitor with charge on its free surface and an equal charge of opposite polarity on the conductive substrate. A light image is then projected onto the photoconductive layer. The portions of the layer that are not exposed to light retain their surface charge. After development of the latent image with toner particles to form a toner image, the toner image is usually transferred to a receiving member, such as paper.
Many advanced imaging systems are based on the use of small diameter photoreceptor drums. The use of such small diameter drums places a premium on photoreceptor life. A major factor limiting photoreceptor life in copiers and printers is wear. The use of small diameter drum photoreceptors exacerbates the wear problem because, for example, 3 to 10 revolutions are required to image a single letter size page. Multiple revolutions of a small diameter drum photoreceptor to reproduce a single letter size page can require up to 1 million cycles from the photoreceptor drum to obtain 100,000 prints, a desirable goal for commercial systems.
For low volume copiers and printers, bias charging rolls (BCR) are desirable because little or no ozone is produced during image cycling. However, the micro corona generated by the BCR during charging damages the photoreceptor, resulting in rapid wear of the imaging surface, e.g., the exposed surface of the charge transport layer. For example, wear rates can be as high as about 16 micrometers per 100,000 imaging cycles. Similar problems are encountered with bias transfer roll (BTR) systems.
One common type of photoreceptor is a multi-layered device that comprises a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. The charge transport layer may contain an active aromatic diamine molecule, which enables charge transport, dissolved or molecularly dispersed in a film forming binder. A charge transport layer of this type is disclosed in U.S. Pat. No. 4,265,990, the disclosure of which is incorporated herein by reference. Another type of charge transport layer has been developed that employs a charge transporting polymer wherein the charge transporting moiety is incorporated in the polymer as a group pendant from the backbone of the polymer backbone or as a moiety in the backbone of the polymer. These types of charge transporting polymers include poly (N-vinylcarbazole), polysylenes, and others.
One approach to achieving longer photoreceptor drum life is to form a protective overcoat on the imaging surface, e.g. the charge transporting layer of a photoreceptor. This overcoat layer must satisfy many requirements, including transporting holes, resisting image deletion, resisting wear and avoidance of perturbation of underlying layers during coating. Although various hole transporting small molecules can be used in overcoating layers, one of the toughest known overcoatings includes cross-linked polyamide (e.g. Luckamide) containing N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-hydroxyphenyl)-[1,1xe2x80x2-biphenyl]-4,4xe2x80x2-diamine. This overcoat is described in U.S. Pat. No. 5,368,967, the entire disclosure thereof being incorporated herein by reference.
Durable photoreceptor overcoatings containing cross-linked polyamide (e.g. Luckamide) and N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-hydroxyphenyl)-[1,1xe2x80x2-biphenyl]-4,4xe2x80x2-diamine (DHTBD) have been prepared using oxalic acid and trioxane to improve photoreceptor life by at least a factor of 3 to 4. The improved wear resistance involved cross-linking of Luckamide under heat treatment, e.g. 110xc2x0 C.-120xc2x0 C. for about 30 minutes. However, adhesion of this overcoat to certain photoreceptor charge transport layers, containing certain polycarbonates (e.g., Z-type 300) and charge transport materials [e.g., bis-N,N-(3,4-dimethylphenyl)-N-(4-biphenyl)amine and N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-[1,1xe2x80x2-biphenyl]-4,4xe2x80x2-diamine] is greatly reduced under such drying conditions. On the other hand, under drying conditions of below about 110xc2x0 C., the overcoat adhesion to the charge transport layer was good, but the overcoat had a high rate of wear. Thus, there was an unacceptably small drying condition window for the overcoat to achieve the targets of both adhesion and wear rate.
One known long-life overcoat formulation depends upon acid catalyzed condensation of N-methoxy-methyl groups and Nxe2x80x94H units. This overcoat formulation can be a self-condensation of Luckamide, which contains both units, or a cross-linking agent, such as hexamethoxymethylmelamine (commercial name Cymel 303) plus Luckamide or Elvamide (the latter two materials being alcohol soluble nylon polyamides). While these formulations have beneficial wear properties, they suffer from certain drawbacks, including limited pot life. In addition, the acid catalyst at optimum concentration can degrade the electrical properties of the photoreceptor layers. Moreover, there is no effective method to chemically bond surface energy reducing components into the overcoat composition to improve the performance of the overcoat with certain toners.
In spite of the many improvements, there remains an urgent need for an effective, wear resistant overcoat. Since the drums are typically dip coated, one of the requirements for the overcoat material is ease and economical synthesis of materials and a coating solution pot life of several weeks. Pot life is the life of the coating composition without changes in its properties so that the same mixture can be used for several weeks. With coating compositions that ultimately cross-link and provide wear protection, there is a danger of initiation of cross-linking in the pot itself rendering the remaining material in the pot useless. Since the unused material must be discarded and the pot cleaned or replaced, this waste of material and effort has a significant negative impact on the manufacturing cost.
In U.S. Pat. No. 5,702,854 to Schank et al., issued Dec. 30, 1998, an electrophotographic imaging member is disclosed including a supporting substrate coated with at least a charge generating layer, a charge transport layer and an overcoating layer, said overcoating layer comprising a dihydroxy arylamine dissolved or molecularly dispersed in a cross-linked polyamide matrix. The overcoating layer is formed by cross-linking a cross-linkable coating composition including a polyamide containing methoxy methyl groups attached to amide nitrogen atoms, a cross-linking catalyst and a dihydroxy amine, and heating the coating to cross-link the polyamide. The electrophotographic imaging member may be imaged in a process involving uniformly charging the imaging member, exposing the imaging member with activating radiation in image configuration to form an electrostatic latent image, developing the latent image with toner particles to form a toner image, and transferring the toner image to a receiving member.
U.S. Pat. No. 5,681,679, issued to Schank, et al. on Oct. 28, 1997, discloses a flexible electrophotographic imaging member including a supporting substrate and a resilient combination of at least one photoconductive layer and an overcoating layer, the at least one photoconductive layer comprising a hole transporting arylamine siloxane polymer and the overcoating comprising a cross-linked polyamide doped with a dihydroxy amine. This imaging member may be utilized in an imaging process including forming an electrostatic latent image on the imaging member, depositing toner particles on the imaging member in conformance with the latent image to form a toner image, and transferring the toner image to a receiving member.
It is, therefore, an object of the present invention to provide overcoat compositions with longer pot life. It is another object of the invention to provide overcoat compositions that do not require an acid catalyst.
Yet another object of the present invention to provide overcoat compositions that include surface energy reducing agents. It is another object of the invention to provide overcoats that resist wear.
The foregoing objects and others are accomplished in accordance with this invention by providing a composition for coating a photoreceptor having a charge generating layer and a charge transport layer, the composition comprising a hole transport material; a cross-linkable film forming binder having at least one functional group that is reactive with isocyanate; a blocked isocyanate cross-linking agent that is the reaction product of an isocyanate and a blocking agent; wherein the blocking agent has a boiling point temperature equal to or below a selected deblocking temperature to allow the isocyanate to form cross-links; and a solvent having a boiling point equal to or below the deblocking temperature. In addition, the compositions of this invention optionally include a surface energy reducing agent.
The invention also provides an electrophotographic imaging member comprising a substrate; a charge generating layer; a charge transport layer; and an overcoat layer. The overcoat layer includes a cross-linked film forming binder, an isocyanate compound, a hole transport material and optionally, a cross-linked surface energy reducing agent.
The electrographic imaging member of this invention may be fabricated by forming a coating solution according to this invention; forming a coating with the coating solution on a photoreceptor having a charge generating layer and a charge transport layer, and heating the coating to the deblock temperature, the deblock temperature equal to or higher than a boiling point of the solvent to form an overcoat layer.